Method and apparatus for venting a storage vessel

ABSTRACT

A method and apparatus for venting volatile organic compound vapors from a liquid storage vessel which includes the steps of introducing a purge medium to a liquid storage vessel containing volatile organic compound vapors and establishing an uniform and continuous stratified interface between the purge medium and the volatile organic compound vapors. The introduction of the purge medium is continued causing the continuous stratified interface to move within the vessel purging the undiluted volatile organic compound vapors from the vessel and into a vapor recovery line which delivers the volatile organic compound vapors to a vapor handling device. Preferably, the purge medium comprises carbon dioxide or nitrogen.

This is a continuation, of application Ser. No. 08/117,397 filed Sep. 3,1993, now U.S. Pat. No. 5,377,723.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method and apparatus for ventingresidual vapors from a liquid storage vessel. More particularly, thepresent invention relates to a method and apparatus for venting residualvapors from a liquid storage vessel by introducing a gas to the storagevessel after removal of all liquids to provide a motive force to ventthe vapors.

2. Description of the Prior Art

Volatile liquids, such as benzene, petroleum and the like, are oftenstored in tanks at bulk terminals, refineries and end-user facilities,and transported in tanks aboard barges or ships, tank trucks and railcars. All such containers shall be referred to herein as liquid storagevessels. While resident in these liquid storage vessels, volatilizationof the liquid occurs leaving residual vapors which must be removedbefore workmen can be permitted to enter the vessel and before thevessel can be filled with a different liquid.

Currently, such residual vapors are purged by flooding liquid storagevessels with a sufficient volume of water or air to entrain the vaporsand carry them out of the vessel. The resulting mixture of dilutedvapors, in many cases, are simply emitted to the atmosphere andsurrounding water supply where they pollute the environment. Emissionshandled in this manner lead to severe environmental hazards. Forexample, the inhalation of benzene vapors may cause depression of bonemarrow activity, convulsions and paralysis. In addition, hydrocarbonsare a major contributor to the formation of smog which has been provento increase respiratory disorders among the population.

In addition to these environmental problems, water flushing facilitiesmust overcome many economic hurdles. Adequate water for such facilitiesmay be expensive due to limited water resources or to restrictionsconcerning the reuse or recycling of the water. If the water must bereused or recycled, it must be treated to remove contaminants that mightpollute the environment or contaminate the next vessel to be flushed.

The environmental problems associated with air flushing could beeliminated by sending the mixture of vented air and vapors to acombustion device where the harmful vapors would be destroyed ratherthan emitted to the atmosphere. Unfortunately, as much as three timesthe liquid storage vessel volume of air must be cycled through thevessel to ensure that all of the residual vapors are purged from thevessel. Clearly, such a solution is impractical because of the largeamount of air which would have to be heated in a combustion devicebefore the volatiles they carry would be destroyed. The size of thecollection piping and combustion equipment associated with such aprocess, in addition to the amount of fuel required to combust thevapors, similarly would be quite large, thereby prohibitively increasingthe cost of such a process.

There have been several patents in the prior art which attempted toaddress the problem of removing vapors from storage tanks and collectingthe gases which are forced out of the storage tank to reuse such gasesfor combustion.

U.S. Pat. No. 291,085 shows apparatus for removing flammable gases fromoil tanks which includes devices for causing an induced current of airto pass into a storage tank above the surface of the fluid (such as fueloil) and at the same time conduct displaced gases to a point where theymay be used as fuel or discharged with safety into the atmosphere. Thepatent which issued in 1884 teaches the use of air as a medium forforcing gaseous vapors from a storage tank.

It has been learned over the past hundred years that air is an unsafemedium for use in cleansing storage tanks and also can result incorrosion of the tank. The device shown by the patent is relativelysimple and primitive and does not include the safety features orefficient means for recapture of vapors for other use as is claimed bythe present invention.

U.S. Pat. No. 1,918,100 shows a gas-gathering system which is basicallya closed system in which vapors which collect in a storage tank arepumped into a secondary vapor storage tank partially filled with waterand from the vapor storage tank are recaptured through a compression andcondensing process to provide dry gas for other uses such as combustion.The patent states as its primary objective the provision of a method andapparatus for maintaining a hydrocarbon gas at all times within thestorage tanks above the liquid levels thereof with the specific end inview of preventing air or oxygen from entering the tanks and mixing withthe gases contained therein.

It should be noted at this point that this patent specifically teachesaway from the method and apparatus of the U.S. Pat. No. 291,085 patentin that U.S. Pat. No. 291,085 teaches the use of air as a medium formoving vapors out of a storage tank, and U.S. Pat. No. 1,918,100specifically provides a method to prevent air or oxygen from enteringthe tank and mixing with the gases.

Although the U.S. Pat. No. 1,918,100 patent is a more modern gascollection system apparatus and method, it does not show nor suggest thepresent invention which includes control of the flow of a purge mediumto provide a laminar flow to create a continuous stratified interfacebetween the volatile vapors and the purge medium. Nor does U.S. Pat. No.1,918,100 teach or suggest any mechanism for detection of completion ofthe purging operation nor mixing with a high BTU material for latercombustion. Nor does either prior art patent introduce gas at the bottomof the tank as is shown and claimed with respect to one embodiment ofthe present invention.

SUMMARY OF THE INVENTION

The method and apparatus for venting a liquid storage vessel of thepresent invention overcome the above-noted disadvantages and drawbackswhich are characteristic of the prior art.

The present invention is directed to a method which comprises the stepsof introducing a purge medium to a liquid storage vessel containingvolatile organic compound vapors and establishing a uniform andcontinuous stratified interface between the purge medium and thevolatile organic compound vapors. The introduction of the purge mediumis continued causing the continuous stratified interface to move withinthe vessel purging the undiluted volatile organic compound vapors fromthe vessel.

In a preferred embodiment, a purge medium, preferably carbon dioxide, isintroduced to the bottom of a liquid storage vessel containingrelatively light volatile organic compound vapors establishing a uniformand continuous stratified interface between the purge medium and thevolatile organic compound vapors. The introduction of the purge mediumis continued causing the continuous stratified interface to rise withinthe vessel purging the undiluted volatile organic compound vapors fromthe top of the vessel.

In an alternate preferred embodiment, a purge medium, preferablynitrogen, is introduced to the top of a liquid storage vessel containingrelatively heavy volatile organic compound vapors establishing a uniformand continuous stratified interface between the purge medium and thevolatile organic compound vapors. The introduction of the purge mediumis continued causing the continuous stratified interface to descendwithin the vessel purging the undiluted volatile organic compound vaporsfrom the bottom of the vessel.

In a preferred embodiment, the undiluted volatile organic compoundvapors are purged into a vapor recovery line which delivers the volatileorganic compound vapors to a vapor handling device.

The present invention also is directed to apparatus for performing theabove-described methods.

BRIEF DESCRIPTION OF THE DRAWINGS

Numerous objects, features and advantages of the present invention willbe readily apparent to those of ordinary skill in the art upon a readingof the following disclosure when taken in conjunction with theaccompanying drawings, in which:

FIG. 1 shows a schematic view of apparatus for performing the storagetank venting method of the present invention;

FIG. 2 shows a schematic view of a modified version of the apparatusdepicted in FIG. 1;

FIG. 3 shows a Schematic view of apparatus for performing the storagetank venting method of the present invention; and

FIG. 4 shows a schematic view of a modified version of the apparatusdepicted in FIG. 3.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to the drawings, and particularly to FIG. 1, a schematicrepresentation of a preferred embodiment of a liquid storage vesselventing apparatus arranged to perform the method of the presentinvention is generally shown at 8. A liquid storage vessel 10 of typicalconstruction has apertures for receiving and dispensing various fluids,including volatile organic compounds such as benzene and petroleumproducts which generate vapors 12.

According to this embodiment, a gas 14 is introduced at or near thebottom of the vessel 10 to provide a motive force to purge the vapors 12from the vessel 10. The gas 14 preferably is heavier than and inert withrespect to the vapors 12, and is introduced to the vessel 10 in alaminar, or near laminar, flow with little or no turbulence. As the gas14 enters the vessel 10, an uniform stratification develops with thevapors 12 forming a layer above a layer formed by the gas 14. In thismanner, mixing of the gas 14 with the vapors 12 is avoided and thevapors 12 remain undiluted. As shown in FIG. 1, an uniform interface 16develops between the gas 14 and the vapors 12. As more gas 14 isintroduced to the vessel 10, the interface 16 approaches the top of thevessel 10 driving the undiluted vapors 12 from the vessel 10.Preferably, the gas 14 is such that it may be dispersed into theenvironment without contaminating the surrounding area. Preferably, thegas 14 comprises carbon dioxide. Those of ordinary skill in the art willrecognize that other gases that are heavier than the vapors 12 may alsobe utilized as the gas 14.

A supply of gas 14 is provided in an insulated tank 18. Preferably, thegas 14 is stored in the tank 18 in a chilled liquid state. The gas 14 isdelivered to the vessel 10 through a fill line 20. The fill line 20,preferably, introduces the gas 14 to the vessel 10 via an opening 22disposed near the bottom of the vessel 10. A valve 24 is provided in thefill line 20 to control the flow of the gas 14 from the tank 18.

Preferably, the gas 14 is stored as a liquid under high pressure. Thegas 14 undergoes a pressure reduction as it leaves the tank 18 causingthe gas to auto-refrigerate in the fill line 20. A heat exchanger 26 isprovided to interact thermally with the gas 14 as it passes through thefill line 20 to raise the temperature of the gas 14 to a desired point,preferably to about 0° F.

As shown in FIG. 2, the gas 14 may alternately be delivered to thevessel 10 through a fill line 20, which extends through an opening 22,disposed on the top of the vessel 10. The distal end of the fill line20, is disposed near the bottom of the vessel 10 to introduce the gas 14beneath the vapors 12.

Referring to both FIGS. 1 and 2, a vapor recovery line 28 extends froman opening 30 disposed in the roof of the vessel 10 to receive thevapors 12 that are purged from the vessel 10 as it is filled with thegas 14. A gas detector 32 may be provided in the recovery line 28 tomonitor the gas purged from the vessel 10 to check for the presence ofthe gas 14. When the gas 14 is detected by the gas detector 32, theventing process has been completed.

A low pressure blower 34 may be provided to receive the Vapors 12exiting detector 32 and to direct the vapors 12 to a vapor handlingdevice described below. To prevent the vessel 10 from collapsing, caremust be taken to prevent the blower 34 from creating an excessive vacuumwithin the vessel 10.

The vapors 12 may be directed by the blower 34 through a filter 36 forextracting condensed liquids from the vapors 12 flowing through therecovery line 28. After passing through the filter 36, the vapors 12 maybe passed to a vapor handling device 38 where the vapors 12 areprocessed for further handling. Preferably, the vapors are combusted anddestroyed prior to their emission to the atmosphere. If thecombustibility of the vapors 12 is insufficient for adequate burning, asupply of natural gas, propane, butane or other high BTU source materialmay be provided under the control of a valve to increase the BTU levelof the vapors 12. Alternatively, the vapors 12 can be condensed orcooled to a liquid form by a refrigeration system. The recovered liquidscan be deposited in a storage tank for further handling. The vapors 12can also be sent to a system in which the vapors 12 are compressed andrefrigerated for further handling. Another possibility is that, thevapors 12 can be passed through a molecular sieve material that absorbshydrocarbons. Those of ordinary skill in the art will recognize thatother vapor handling devices may be utilized as the circumstances maydictate.

The gas 14 which now fills the vessel 10 may be dispersed from thevessel 10 directly to the atmosphere Either immediately, or upon therefilling of the vessel 10.

Referring now to FIG. 3, a schematic of another preferred embodiment ofthe present invention is shown and referred to in general by thereference numeral 40. A liquid storage vessel 42 of typical constructionhas apertures for receiving and dispensing various fluids, includingvolatile organic compounds such as benzene and petroleum products whichgenerate vapors 44.

According to this embodiment, a gas 46 is introduced at or near the topof the vessel 42 to provide a motive force to purge the vapors 44 fromthe vessel 42. The gas 46 preferably is lighter than and inert withrespect to the vapors 44, and introduced to the vessel 42 in a laminar,or near laminar, flow with little or no turbulence. As the gas 46 entersthe vessel 42, an uniform stratification develops with the gas 46forming a layer above a layer comprised of the vapors 44. In thismanner, mixing of the gas 46 with the vapors 44 is avoided and thevapors 44 remain undiluted. As shown in FIG. 3, an uniform interface 48develops between the gas 46 and the vapors 44. As more gas 46 isintroduced to the vessel 42, the interface 48 approaches the bottom ofthe vessel 42 driving the undiluted vapors 44 from the vessel 42.Preferably, the gas 46 is such that it may be dispersed into theenvironment without contaminating the surrounding area. Preferably, thegas 46 comprises nitrogen, but those of ordinary skill in the art willrecognize that other gases that are lighter than the vapors 44 may alsobe utilized as the gas 46.

A supply of gas 46 is provided in an insulated tank 50. Preferably, thegas 46 is stored in the tank 50 in a chilled liquid state. The gas 46 isdelivered to the vessel 42 through a fill line 52. The fill line 52,preferably, introduces the gas 46 to the vessel 42 via an opening 54disposed near the top of the vessel 42. A valve 56 is provided in thefill line 52 to control the flow of the gas 46 from the tank 50.

Preferably, the gas 46 is stored as a liquid under high pressure. Thegas 46 undergoes a pressure reduction as it leaves the tank 50 causingthe gas to auto-refrigerate in the fill line 52. A heat exchanger 58 isprovided to interact thermally with the gas 46 as it passes through thefill line 52 to raise the temperature of the gas 46 to a desired point,preferably to about 0° F.

A vapor recovery line 60 extends from an opening 62 disposed near thebottom of the vessel 42 to receive the vapors 44 that are purged fromthe vessel 42 as it is filled with the gas 46. Alternatively, and asshown in FIG. 4, a vapor recovery line 60, extends into the vessel 42through an opening 62, disposed on the top of the vessel 42. Theproximal end of the recovery line 60, is disposed near the bottom of thevessel 42 to receive the vapors 44 as the vessel 42 fills with the gas46.

Referring to both FIGS. 3 and 4, a gas detector 64 may be provided inthe recovery line 60 or 60, to monitor the gas purged from the vessel 42to check for the presence of the gas 46. When the gas 46 is detected bythe gas detector 64, the venting process has been completed.

A low pressure blower 66, a filter 68, and a vapor handling device 70may be provided to receive the vapors 44 exiting the detector 64 toprocess them for further handling. Since these devices are the same asthe corresponding devices described in connection with the previousembodiment, they will not be described here in detail.

The present invention will be further illustrated by the followingspecific examples, it being understood that while these examples maydescribe in detail some of the preferred features of the invention, theyare merely provided for the purpose of illustration and are not intendedto limit the broader aspects of the present invention.

EXAMPLE 1

A test of the present invention was conducted to vent the gaseouscontents of a liquid storage vessel carried aboard a barge. For thepurpose of this test, volatile organic compound vapors were removed fromthe tank so that it contained only air.

Liquid carbon dioxide stored in a refrigerated tank truck at about about0° F. and about 300 psia was released into a fill line, which reducedthe pressure of the gas from 300 psia to about 100 psia causing thecarbon dioxide gas to auto-refrigerate to about -35° F. The temperatureof the carbon dioxide gas was raised to about 30° F. by passing the gasthrough a steam heat exchanger. From this point the pressure of thecarbon dioxide within the fill line was dropped to atmospheric pressurewithout forming solids. This pressure reduction, however, again causedthe carbon dioxide gas to auto-refrigerate to about 0° F.

The carbon dioxide gas was then introduced to the bottom of the vesselthrough an 8 inch line at a flow rate of approximately 7,000 cfh. It wasadmitted in a nonturbulent, metered flow to create an even and uniformstratification between the air present in the vessel and the incomingcarbon dioxide gas. An air-carbon dioxide gas interface formed in thevessel and continually rose toward the roof of the vessel as more carbondioxide gas was admitted. As the interface rose, the air in the vesselwas forced toward and through an opening in the roof of the vesselwithout experiencing any significant mixing of the air and carbondioxide gas, leaving the vessel completely void of air and full ofcarbon dioxide gas.

EXAMPLE 2

All liquids are removed from a liquid storage vessel carried aboard abarge. The liquid storage vessel contains volatile organic compoundvapors.

Liquid carbon dioxide stored in a refrigerated tank truck at and about0° F. and about 300 psia is released into a fill line, which reduces thepressure of the gas from 300 psia to about 100 psia causing the carbondioxide gas to auto-refrigerate to about -35° F. The temperature of thecarbon dioxide gas is raised to about 30° F. by passing the gas though asteam heat exchanger. From this point the pressure of the carbon dioxidewithin the fill line is dropped to atmospheric pressure without formingsolids. This pressure reduction, however, again causes the carbondioxide gas to auto-refrigerate to about 0° F.

The carbon dioxide gas is then introduced to the bottom of the vesselthrough an 8 inch line at a flow rate of approximately 7,000 cfh. It isadmitted in a nonturbulent, metered flow to create an even and uniformstratification between the volatile organic compound vapors present inthe vessel and the incoming carbon dioxide gas. A volatile organiccompound vapor-carbon dioxide gas interface forms in the vessel andcontinually rises toward the roof of the vessel as more carbon dioxidegas is admitted. As the interface rises, the air in the vessel is forcedtoward and through an opening in the roof of the vessel withoutexperiencing any significant mixing of the volatile organic compoundvapors and carbon dioxide gas, leaving the vessel completely void ofvolatile organic compound vapors and full of carbon dioxide gas.

The purged volatile organic compound vapors, undiluted by employing thepresent invention, are forced into a recovery line connected to thevessel. The carbon dioxide gas is introduced into the vessel until adetector in the recovery line detects the presence of the carbon dioxidegas. At this point, due to the uniform stratification maintained withinthe vessel, all of the residual vapors originally in the vessel arepurged from the vessel, and the vessel is filled solely with carbondioxide gas. A blower attached to the recovery line directs the vaporsthrough a filter to remove any condensed liquids within the vaporstream. The blower also directs the vapors to a combustion device where,if needed, the vapors are mixed with natural gas or other high BTUsource and combusted prior to their emission to the atmosphere.

It is thus seen that the method and apparatus of the present inventionprovides several advantages. In general, the present invention reducesthe amount of purge medium necessary to vent residual vapors from alltypes of liquid storage vessels to nearly a single vessel volume. Inaddition, since the present invention eliminates the need for flushingwater or air, it is an environmentally safe and efficient way to ventresidual vapors. Further, since the vapors are not diluted, thecombustibility of the vapors might be sufficiently high for burning, orat the very least they can be combusted with a minimal addition of fuel.The present invention can also be used to purge residual vapors that areeither heavier or lighter than the purge medium.

It is understood that variations of the foregoing can be made within thescope of the present invention. For example, numerous purging mediumscan be used to provide the motive force to vent the vessel 10 or 42.Further, the present invention can be used to vent more than justvolatile organic compound vapors from liquid storage vessels. It isapplicable for venting any type of gaseous fluid from any type ofenclosure.

Further, the gas detector can be replaced by a flow meter which measuresthe amount of purging medium introduced into the vessel. A singleturnover of the vessel volume plus 10% extra gas will sufficiently ventthe vapors from the vessel. Since the volume flow rate of the gas andthe volume of the vessel are known, the flow meter can also beeliminated, by calculating and using the time needed to introduce enoughgas to equal 1.1 times the volume of the vessel. In addition, the blowercan be eliminated if the structural design of the vessel is sufficientlyhigh to allow for pushing the vented vapors through the collectionpiping.

A latitude of modification, change and substitution is intended in theforegoing disclosure and in some instances some features of theinvention will be employed without a corresponding use of otherfeatures. Accordingly, it is appropriate that the appended claims beconstrued broadly and in a manner consistent with the scope of theinvention.

What is claimed is:
 1. A method for venting vapors from a storagevessel, comprising the steps of:(i) connecting a fill line to a storagevessel containing vapors; (ii) connecting a vapor recovery line to saidvessel; (iii) releasing a gas into said fill line; (iv) reducing thepressure on said gas to auto-refrigerate said gas; and (v) introducingsaid gas at a controlled temperature, flow rate and pressure to saidvessel through said fill line to maintain a substantially laminar flowof said gas to displace said vapors from said vessel and into said vaporrecovery line.
 2. The method of claim 1 wherein said gas is lighter thansaid vapors.
 3. The method of claim 1 wherein said gas is heavier thansaid vapors.
 4. The method of claim 1 wherein said introduction of saidgas to said vessel is stopped when said gas is detected in said vaporrecovery line.
 5. The method of claim 1 wherein said introduction ofsaid gas to said vessel is stopped when said vessel is filled with saidgas.
 6. The method of claim 1 wherein said gas is inert with respect tosaid vapors.
 7. The method of claim 1 further comprising the step ofburning said displaced vapors in a combustion device.
 8. The method ofclaim 7 further comprising the step of mixing said displaced vapors witha high BTU source material to increase the combustibility of saiddisplaced vapors.
 9. The method of claim 1 further comprising the stepof removing condensed liquids from said displaced vapors.
 10. Apparatusfor venting vapors from a storage vessel, comprising:(i) a storagevessel containing vapors; (ii) a fill line connected to said vessel;(iii) a vapor recovery line connected to said vessel; (iv) means forreleasing a purge medium into said fill line; and (v) means for droppingthe pressure of said purge medium; (vi) means for introducing said purgemedium into said vessel through said fill line at a controlled rate,temperature and pressure to maintain a substantially laminar flow ofsaid purge medium and establish a stratification between said vapors andsaid purge medium.
 11. The apparatus of claims 10 wherein said fill lineconnects to the bottom of said vessel and said vapor recovery lineconnects to the top of said vessel.
 12. The apparatus of claim 10wherein said fill line connects to the top of said vessel and said vaporrecovery line connects to the bottom of said vessel.
 13. The apparatusof claim 10 wherein said fill line connects to the top of said vesseland further comprises means for passing said purge medium to the lowerportion of said vessel.
 14. The apparatus of claim 10 wherein said vaporrecovery line connects to the top of said vessel and further comprisesmeans for receiving said vapors from the lower portion of said vessel.15. The apparatus of claim 10 wherein said purge medium is lighter thansaid vapors.
 16. The apparatus of claim 10 wherein said purge medium isheavier than said vapors.
 17. The apparatus of claim 10 furthercomprising means for detecting said purge medium in said vapor recoveryline.
 18. The apparatus of claim 10 wherein said purge medium is inertwith respect to said vapors.
 19. The apparatus of claim 10 furthercomprising means for burning said displaced vapors.
 20. The apparatus ofclaim 19 further comprising means for mixing said displaced vapors witha high BTU source material to increase the combustibility of saiddisplaced vapors.
 21. The apparatus of claim 19 further comprising meansfor removing condensed liquids from said displaced vapors.
 22. A methodfor venting vapors from a storage vessel, said method comprising thesteps of:(i) connecting a fill line to a storage vessel containingvapors; (ii) connecting a vapor recovery line to said vessel; (iii)releasing a gas into said fill line; (iv) heating said gas; (v) reducingthe pressure on said gas; and (vi) introducing said gas at a controlledflow rate, pressure, and temperature into said vessel through said fillline to establish and maintain a substantially laminar flow of said gasto displace said vapors from said vessel and into said vapor recoveryline until said gas is detected in said vapor recovery line.
 23. Anapparatus for venting vapors from a storage vessel, said apparatuscomprising:(i) a storage vessel containing vapors; (ii) a fill lineconnected to said vessel; (iii) a vapor recovery line connected to saidvessel; (iv) means for releasing a purge medium into said fill line; (v)means for heating said purge medium; (vi) means for dropping thepressure of said purge medium; (vii) means for introducing said purgemedium into said vessel at a controlled flow rate, pressure andtemperature to establish and maintain a substantially laminar flow ofsaid purge medium to displace said vapors from said vessel; and (viii)means for detecting said purge medium in said vapor recovery line.